The present invention relates to a continuous process for the manufacture of methionine [.alpha.-amino-.gamma.-methylmercaptobutyric acid].
Methionine has already been made by hydrolysis of 5-(.beta.-methylmercaptoethyl)-hydantoin. The hydrolysis is carried out by heating the hydantoin, possibly under pressure, in the presence of water and specific alkaline materials. There is thus formed a solution of a methionine salt from which the pure methionine is obtained by precipitation when neutralizing the solution with an acid.
The hydrolysis has also been carried out in the reverse manner, that is by means of an acid while the subsequent precipitation of the methionine was effected by neutralization with alkali.
The alkali used in these processes were alkali- or earth alkali hydroxides such as sodium or barium hydroxide and the acids were sulphuric acid or hydrochloric acid (British patent No. 630,139).
It has also been proposed to use as alkali sodium or potassium carbonate and to use as acid a lower aliphatic carboxylic acid such as acetic acid (German patent No. 891,259).
Another proposal has been to effect the reaction of 5-alkyl substituted hydantoins by means of alkali mixtures of alkali carbonate and alkali hydroxide and to use carbon dioxide as the acid (Japanese published application 19530/68).
During the hydrolysis there are formed gaseous products. A more recent proposal is to effect the removal of these gaseous products during the reaction from the system in order to increase the yield of methionine (Netherland published application No. 65 16950).
In all these processes there occurs a substantial amount of unwanted salts formed in the neutralization which depending on the type of alkalis and acids may for instance be alkali- or earth alkali sulfates, alkali chlorides, alkali acetates or alkali bicarbonates. The separation of the methionine and the purification from these unwanted salts which may be present in a multiple molar excess causes substantial difficulties particularly if the methionine is to be obtained in a high yield.
Another shortcoming of the prior art processes is the fact that there is a continuous using up of alkali and acid. The formed salts are hardly useful. Their separation from the mother liquor is normally not worth the effort and they are therefore usually discarded together with the mother liquor. This results in substantial amounts of sewage water.
Together with the mother liquor there are also always lost certain amounts of methionine.
It is therefore an object of the present invention to avoid these shortcomings of the prior art.